#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
#define rep(i, a, b) for (int i = (a); i <= (b); i++)
#define per(i, a, b) for (int i = (a); i >= (b); i--)
#define endl '\n'
const int N = 1e4 + 5;

int n, s;
int c[N], y[N];

int main() {
  ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  cin >> n >> s;
  rep(i, 1, n) cin >> c[i] >> y[i];
  ll sum = 0;
  rep(i, 1, n) {
    sum += 1ll * c[i] * y[i];
    c[i + 1] = min(c[i + 1], c[i] + s);
  }
  cout << sum;
  return 0;
}